Since commit aa7a6487
("net: Stop including NFS overhead in defragment max") the following
has been reproducibly observed while trying to transfer data over TFTP:

Load address: 0x80408000
Loading: EHCI timed out on TD - token=0x8008d80
T EHCI timed out on TD - token=0x88008d80
Rx: failed to receive: -5

This patch fixes this by lowering our TFTP block size to be within the
standard maximal de-fragmentation aka IP packet size again.

Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>